He didn't throw weak attacks, that doesn't make sense knowing that someone is strong. But at the same time he couldn't really be sure that Lucci was worth Gear 2.
This whole thing goes to two points: that he doesn't have fights that last for days, and that people probably think it was weird the way he approached Kaido. It's rare that Luffy makes his FIRST move his hardest, like the time he told Aokiji to fight him. Not so he can do weak shit, but he's not so irrational that he doesn't pace himself at all. But he'll try really hard early on if he thinks it's necessary, as he showed against Big Mom, Cracker, all throughout the raid, etc.
If you add up every fight he's had, it probably doesn't equal five days even if he's way stronger than Ace or Jack qt this point. Like an actual fighter he has the stamina, but he'd much rather win sooner than later, unlike Queen or Kaido for example.
Luffy's proven to be reckless all throughout the series so I don't know why you need that explained.
But because you mentioned a contradiction with his early gear shifts: 2 and 3 are interchangeable. 3 has monster power but average speed, 2 has pretty good power and high speed. Using 2 against Lucci wasn't just holding back 3 because of the drawback, the opponent was super fast and so he used super speed. You have to realize 3 as a finisher applies to guys like Caesar and Chinjao for the most part, lol.
Anyway, here's me actually proving what I say
He'll go from 2 to 3 or 3 to 2 based on what he needs. Gear 2 doesn't mean he's holding back.